The present invention generally relates to a convergence apparatus and a convergence yoke to be used for the convergence apparatus. The present invention particularly relates to a convergence apparatus in which it is possible to prevent an electron beam spot from deteriorating at the time of convergence correction to thereby provide a good focusing performance, and a convergence yoke to be used for such a convergence apparatus.
In a color display of a projection type color television receiver, a plurality of single-electron-gun type cathode-ray tubes are used and monochromatic images of red (R), green (G), and blue (B) of the respective cathode-ray tubes are projected on a projection screen through an optical system composed of a reflection mirror, a lens, etc., so as to form a color picture on the screen. At this time, in each cathode-ray tube, an electron beam is deflected by a substantially even magnetic field type deflection yoke and image carrying light of the cathode-ray tube is projected on a projection screen 101 (FIGS. 1 and 2) through an optical system, so that a pincushion distortion 102 as shown in FIG. 1 or a trapezoidal or keystone distortion 103, 103' as shown in FIG. 2 appears on the projection screen 101.
In order to correct such a distortion, conventionally, a convergence apparatus has been provided in each cathode-ray tube. Such a convergence apparatus is constituted by a convergence yoke and a convergence circuit, the convergence yoke being provided in the electron-gun side rear of a deflection yoke. The convergence yoke has a core composed of a ring-like portion and four or two-pairs of rectangular core protrusions, each pair being located on the horizontal and vertical axes respectively and inward projected from the ring-like portion. The convergence circuit makes a convergence correction current i.sub.c flow into coils wound on the core protrusions in synchronism with a horizontal deflection current i.sub.H or a vertical deflection current i.sub.v as shown in FIG. 3 so as to generate a horizontal bipolar magnetic field between the one pair of core protrusions (magnetic poles) located on the horizontal axis at positions opposite to each other and a vertical bipolar magnetic field between the other pair of core protrusions (magnetic poles) located on the vertical axis at positions opposite to each other, so that vertical and horizontal deflection forces are exerted on the electron beam to correct the above-mentioned pincushion distortion 102 or the keystone distortion 103, 103' .
Alternatively, as is shown in FIG. 6 of Japanese Utility Model Publication No. 58-32378, there has been proposed another example of a core of a convergence yoke in which an 8-pole core having two pairs of a first set of four core protrusions located on the horizontal and vertical axes respectively and a second set of four core protrusions positioned at circumferentially intermediate angular positions between adjacent ones of the first set of four core protrusions, and in which an AC current is made to flow in coils wound on the first set of four core protrusions to perform dynamic convergence correction (that is, correction of the foregoing pincushion distortion 102 or the keystone distortion 103, 103') and a DC current is made to flow in coils wound on the second set of four core protrusions so as to perform static correction.
In the prior art, as described above, there have been two examples of a convergence yoke of a convergence apparatus, one example being a case .circle.1 in which horizontal and vertical bipolar magnetic fields are generated by two pairs of core protrusions positioned on the horizontal and vertical axes respectively, the other example being a case .circle.2 in which horizontal and vertical bipolar magnetic fields are generated by four core protrusions positioned at circumferentially intermediate angular positions between the horizontal and vertical axes (that is, positions angularly deviated, for example, by 45.degree. or the like from the horizontal and vertical axes). The respective shapes of the horizontal and vertical bipolar magnetic fields in the foregoing cases, that is, the shapes of magnetic fields acting on an electron beam, will be described hereunder.
First, the former case .circle.1 will be described by using FIG. 4.
FIG. 4 is a sectional view showing a example of a convergence yoke in a conventional convergence apparatus.
In FIG. 4, reference numerals 6 and 7 designate vertical and horizontal bipolar magnetic fields (N, S) respectively; 15b and 15b' designate horizontal deflection coils; 16a and 16a' designate vertical deflection coils; 17, 17' and 18, 18' designate input terminals; 50 designates a core; and a, a' and b, b' designate core protrusions.
As is apparent from FIG. 4, in the case where a bipolar magnetic field in the horizontal direction (x-direction) is generated by the pair of core protrusions a and a' provided on the horizontal axes and another bipolar magnetic field in the vertical direction (y-direction) is generated by the pair of core protrusions b and b' provided on the horizontal axes, the qualitative shape of each of the resultant bipolar magnetic fields shows a section of a glass tube through which the electron beam passes.
Since the convergence yoke has a structure symmetrical with respect to the x-axis as well as the y-axis as shown in FIG. 4, description will be made hereunder only about the vertical bipolar magnetic field (the horizontal convergence correcting magnetic field B.sub.y).
The horizontal convergence correcting magnetic field B.sub.y is expressed by the following expression (1). EQU B.sub.y =B.sub.0 +B.sub.2 .multidot.x.sub.2 ]10.sup.-4 T]. . . (1)
where x represents an amount of deviation in the horizontal direction (x-direction) from a reference, that is, a tube axis (an axis perpendicular to the paper plane and passing through the intersection of the horizontal and vertical axes in FIG. 4), B.sub.0 represents the magnetic flux density on the tube axis, and B.sub.2 represents a value expressed as follows. ##EQU1## where B(x) represents the magnetic flux density at a position deviated in the horizontal direction by x from the tube axis, and B(-x) represents the magnetic flux density at a position deviated in the horizontal direction by -x from the tube axis.
Although actual magnetic flux density contains higher order components of even number degree four or more (x.sup.4, x.sup.6, x.sup.8, . . . ) with respect to x, those components are omitted in the above expression (1).
In the above expression (1), B.sub.2 represents an uneven magnetic field component of the vertical bipolar magnetic field 6.
FIG. 5 is an explanatory diagram showing, along a tube axis, a distribution of an uneven magnetic field component of a bipolar magnetic field generated in the conventional convergence yoke of FIG. 4.
In FIG. 5, the abscissa represents the coordinate Z in the direction of tube axis (in the direction perpendicular to the paper plane in FIG. 4), and the axis of ordinates represents the value of B.sub.2 /B.sub.0max which is obtained by normalizing the uneven magnetic field component B.sub.2 with the maximum value B.sub.0max of the magnetic flux density B.sub.0 on the tube axis. The reference numeral 13 represents the distribution curve of the value of B.sub.2 /B.sub.0max, and 14 represents the position, on the tube axis, of the core 50 of the convergence yoke in FIG. 4. In this case, it is considered that the distribution curve of the value of B.sub.2 /B.sub.0max may take the following three states (a), (b) and (c).
(a) The case where both the B.sub.2 and B.sub.0 take positive values:
B.sub.2 &gt;0 and B.sub.0 &gt;0 so that B.sub.2 /B.sub.0max takes a value in the positive region in FIG. 5 and therefore B.sub.2 /B.sub.0 &gt;0 and B.sub.0 &lt;B(x). Accordingly, the magnetic field becomes a pincushion magnetic field.
(b) The case where B.sub.2 takes a negative value, while B.sub.0 takes a positive value:
B.sub.2 &lt;0 and B.sub.0 &gt;0 so that B.sub.2 /B.sub.0max takes a value in the negative region in FIG. 5 and therefore B.sub.2 /B.sub.0 &lt;0 and B.sub.0 &gt;B(x). Accordingly, the magnetic field becomes a barrel magnetic field.
(c) The case where B.sub.2 becomes zero:
B.sub.2 =0 and B.sub.2 /B.sub.0 =0, so that B.sub.2 /B.sub.0max takes a value on the abscissa in FIG. 5 and therefore the magnetic field becomes an even one.
As seen from FIG. 5, in the case where horizontal and vertical bipolar magnetic fields are generated by two pairs of core protrusions positioned on the horizontal and vertical axes respectively, the magnetic fields become like a barrel over the whole region in the direction of the tube axis and the barrel shape becomes most remarkable at a central position A of the convergence yoke.
FIG. 6 is a partly broken side view of a cathode-ray tube and peripheral devices and FIG. 7 is an explanatory diagram showing a state in which a shape of an electron beam spot is distorted by a barrel magnetic field component of a bipolar magnetic field generated in the convergence yoke of FIG. 4.
In FIG. 6, the reference numeral 32 represents a cathode-ray tube, 33 represents a fluorescent screen, 34 represents a horizontal deflection coil, 35 represents a vertical deflection coil, 36 represents a deflection yoke core, 37 represents a deflection yoke, 38 represents a deflection circuit, 39 represents a convergence yoke, 40 represents a convergence circuit, 41 represents a centering magnet, 42 represents a cathode-ray tube wall, and 43 represents an electron gun.
As shown in FIG. 6, the convergence yoke 39 is positioned in the electron-gun side rear of the deflection yoke 37 so that a bipolar magnetic field in the vertical (horizontal) direction is exerted on an electron beam. If such a barrel magnetic field as described above is exerted on an electron beam in this configuration, the shape of a spot 29 of the electron beam is triangularly distorted as indicated with a reference numeral 30 in FIG. 7 to thereby deteriorate the focusing performance. That is, the reference numerals 29 and 30 designate the shape of the spot of an electron beam before and after the electron beam enters the region of the convergence yoke, respectively.
Next, referring to FIG. 8, description will be made about the latter case .circle. 2 in which horizontal and vertical bipolar magnetic fields are generated by four core protrusions positioned at circumferentially intermediate angular positions between the horizontal and vertical axes (that is, positions angularly deviated, for example, by 45.degree. or the like from the horizontal and vertical axes).
FIG. 8 is a sectional view showing another example of a convergence yoke in a conventional convergence apparatus.
In FIG. 8, the reference numerals 24a, 24a' , 24b and 24b' represent horizontal deflection coils; 25, 25' , 26 and 26' input terminals; 27a, 27a', 27b and 27b' represent vertical deflection coils; and 51 represents a core.
As will be apparent from FIG. 8, in the case where horizontal and vertical bipolar magnetic fields 7 and 6 are generated by four core protrusions a, a', b and b' located at positions angularly deviated by 45.degree. from the horizontal and vertical axes respectively, that is, in the case where each magnetic pole is constituted by adjacent two of the core protrusions which are positioned on the both sides of and deviated from each magnetic field symmetry axis and each bipolar magnetic field is generated between each pair of opposite magnetic poles (for example, the vertical bipolar magnetic field 6 is generated between one magnetic pole constituted by the two adjacent core protrusions a and b' located on the both sides of the vertical axis which is the magnetic field symmetry axis and the other magnetic pole constituted by the two adjacent core protrusions a' and b located on the both sides of the same vertical axis), the qualitative shape of each of the bipolar magnetic fields is like a pincushion.
FIG. 9 is an explanatory diagram showing, along a tube axis, a distribution of an uneven magnetic field component of a bipolar magnetic field generated in the conventional convergence yoke of FIG. 8.
As will be apparent in FIG. 9, the B.sub.2 /B.sub.0max distribution curve 13 exists in the positive region so that the magnetic field is a pincushion magnetic field in the certain region from the center of the core 51 in the direction of the tube axis.
The action of the convergence yoke forming such a pincushion magnetic field on an electron beam spot distorts the shape of the electron beam spot from a round one into a triangular one, on the contrary to the case of the foregoing barrel magnetic field, as shown in FIG. 10, resulting in deterioration in focusing performance.